SUMMARY The filovirus family includes Marburg and Ebola viruses, most of which cause highly lethal hemorrhagic fever. The first filovirus was identified when it sickened laboratory workers in Europe in 1967. Since then, filo- viruses have re-emerged multiple times, with modern strains conferring greater lethality (~90%). Ebola virus is typically found in Central Africa, but re-emerged in Western Africa in 2014 to cause an on-going outbreak un- precedented in magnitude and geographic spread that has already claimed the lives of thousands of people. There is also a current outbreak of Marburg virus in Uganda. Last month structural data on several neutralizing antibodies (Abs) in complex with the viral target enve- lope surface glycoprotein (GP) became available to us, enabling for the first time the research proposed within this PAPID proposal. Specifically, co-crystal structures of one Ab with the Marburg and Ebola GP, respectively, as well as electron microscopy (EM) density maps for six additional Abs. It is our Goal to investigate the shared structural determinants of human Abs neutralizing the filovirus. While the present study thereby focuses on the biological question of the immune response to an infection by the Ebola and Marburg viruses, it has the potential to facilitate development of therapeutic strategies by others. Within the scope of this R21 proposal we pursue three goals: Aim 1 creates atomic detail models of the Ab/GP interface for all seven Abs with available EM density maps. We will use the co-crystal structure to test our computational protocol, and then apply it to the remaining six cases. As a result, we will obtain a map of critical structural determinants an Ab needs to fulfill for being neutralizing. Aim 2 will identify novel Abs from the repertoire of nave subjects that are likely to neutralize Ebola and/or Marburg viruses with a limited set of muta- tions. We also will compare the Ab repertoires of filovirus-nave (i.e. not previously infected) humans to those of immune (previously infected) subjects available to us. These Abs from nave subjects are important, as they could evolve into neutralizing Abs upon infection or vaccination. We will redesign these Abs computationally to bind and neutralize Ebola and/or Marburg viruses, to increase the pool of known neutralizing Abs, paving the way for a successful vaccination strategy. In Aim 3 we will characterize these Abs experimentally for binding and neutralization activity. While ultimately a structural characterization of the most promising of these Abs is planned, it is outside the scope of the R21 proposal.